Reapply "Verifier: Check for incompatible bit piece expressions"

This reverts commit r234717, reapplying r234698 (in spirit).

As described in r234717, the original `Verifier` check had a
use-after-free.  Instead of storing pointers to "interesting" debug info
intrinsics whose bit piece expressions should be verified once we have
typerefs, do a second traversal.  I've added a testcase to catch the
`llc` crasher.

Original commit message:

    Verifier: Check for incompatible bit piece expressions

    Convert an assertion into a `Verifier` check.  Bit piece expressions
    must fit inside the variable, and mustn't be the entire variable.
    Catching this in the verifier will help us find bugs sooner, and makes
    `DIVariable::getSizeInBits()` dead code.

llvm-svn: 234776